The heart is the center of a person's circulatory system. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the body organs and pump it to the lungs where the blood gets oxygenated. These pumping functions are accomplished by cyclic contractions of the myocardium (heart muscles). In a normal heart, the sinoatrial node generates electrical impulses called action potentials. The electrical impulses propagate through an electrical conduction system to various regions of the heart to excite the myocardial tissue of these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various portions of the heart to contract in synchrony to result in efficient pumping functions indicated by a normal hemodynamic performance. A blocked or otherwise abnormal electrical conduction system and/or deteriorated myocardial tissue result in an impaired hemodynamic performance, including a diminished blood supply to the heart and the rest of the body.
The hemodynamic performance is modulated by neural signals in portions of the autonomic nervous system. For example, the myocardium is innervated with sympathetic and parasympathetic nerves. Neural activities on these nerves are known to regulate, among other things, heart rate, blood pressure, and myocardial contractility. Autonomic dysfunction is associated with cardiac dysfunctions and poor hemodynamic performance. For example, in heart failure patients, reduced autonomic balance (increase in sympathetic tone and decrease in parasympathetic tone) is known to be associated with left ventricular dysfunction and increased mortality. Examples of other conditions of autonomic dysfunction, collectively termed as dysautonomia, include postural orthostatic tachycardia syndrome (POTS), neurocardiogenic syncope (NCS), pure autonomic failure (PAF), and multiple system atrophy (MSA). Patients having autonomic dysfunction and the associated cardiac dysfunctions can potentially benefit from controlling the autonomic balance. For example, increasing parasympathetic tone and decreasing sympathetic tone may protect the myocardium by controlling adverse remodeling and preventing arrhythmias after myocardial infarction. Patients with bradycardia-tachycardia syndrome, a variant of sick sinus syndrome characterized by alternating periods of slow and rapid heart rate, may also benefit from regulation of autonomic function. For these and other reasons, there is a need for a means to treat autonomic dysfunction or cardiac dysfunction by controlling autonomic balance.